There are several types of such an antenna: for example the antenna may be a single-reflector antenna.
As described in the work by Mr. Nhu BUI HAI, entitled "Antennes micro-ondes" ("Microwave antennas") (published by Masson, 1978), an antenna of this type having its reflector illuminated by a primary source placed at its focus is commonly used in frequency bands above 400 MHz.
Such an antenna comprises a reflector, which is generally circularly symmetrical, and a primary source which is generally of the horn type when the operating wavelength is in the centimeter range, and of the dipole type including a reflector when the operating wavelength is in the decimeter range.
For a circularly symmetrical paraboloidal reflector having a surface tolerance of about .+-..lambda./16, where .lambda. is the working wavelength, and for a horn type of primary source, the efficiency of such an antenna lies in the range 0.45 to 0.55.
One of the main factors having a considerable effect on antenna efficiency lies in loss of gain due to the surface tolerance of the circularly symmetrical paraboloidal reflector. Thus, a surface tolerance of .+-..lambda./16 loses about 0.4 dB and increases the diffuse radiation level by about 15 dB.
The present invention seeks to reduce these effects considerably.
Another such antenna is an antenna having Cassegrain optics.
Antennas having Cassegrain optics with circularly symmetrical reflectors are well known. They comprise a main reflector of the paraboloidal type, a subreflector which is either a hyperboloid or an ellipsoid, and a primary source.
They provide the following performance characteristics:
In co-polarization: PA0 In cross-polarization:
level of first secondary lobe: about -16 dB/maximum; PA1 efficiency: about 0.55 to 0.65; and PA1 far lobe levels: in the range -5 dB to -15 dB below the isotropic level; and PA1 on axis level: about -35 dB; and PA1 maximum level: -22 to -30 dB/maximum. PA1 16 QAM.fwdarw.-22 to -32 dB/maximum PA1 64 QAM.fwdarw.-28 to -38 dB/maximum PA1 256 QAM.fwdarw.-35 to -45 dB/maximum. PA1 about 0.3 dB of gain; PA1 about ten decibels in diffuse radiation levels; PA1 a drop of about 10 dB to 15 dB in cross-polarization level; and PA1 these are achieved using the same primary source.
Assuming that the primary source provides very good performance (e.g. a corrugated type of horn with an exponential profile), then the performance of a Cassegrain antenna depends essentially on the mechanical qualities of the reflectors, i.e.:
the accuracy of the profiles of the main reflector and of the subreflector;
the accuracy of the relative positioning between these two reflectors; and
the shape, quantity, and positioning accuracy of the support arms for the subreflector.
The worse these criteria, the worse the radiating performance of the antenna. Thus, for a profile tolerance .epsilon. relative to the wavelength .lambda., i.e. for a ratio .epsilon./.lambda. of about .+-.1/20, the performance of a Cassegrain antenna having circularly symmetrical reflectors is as specified above.
When only analog radio beams were in use, such performance corresponded to requirements. Now that digital radio beams are being used, cross-polarization performance has become crucial. It is a function, in particular, of the quality of modulation: 4, 16, 64, or 256 quadrature amplitude modulation (QAM).
Thus, for a given form of modulation, there may be a corresponding value of cross-polarization, e.g. as follows:
Consequently, with 64 QAM digital radio beams, there already exists a need to select component parts for the antenna such that the cross-polarization is lower than in existing antennas. And for 256 QAM digital radio beams, the cross-polarization performance of existing antennas is quite unsatisfactory.
In addition, in order to increase the illumination efficiency in a Cassegrain antenna having circularly symmetrical reflectors, attempts are made to obtain amplitude distribution in its aperture which is uniform and equiphase, while continuing to use a primary source which provides tapering illumination. To do this, two new reflector profiles are defined and referred to as being "shaped". The main reflector is a pseudo-paraboloid and the subreflector is a pseudo-hyperboloid. By "shaping" the profile of the subreflector, the illumination of the main reflector is made uniform, and by "shaping" the main reflector, the illumination in the aperture of the antenna is made equiphase. However, when such a "shaped" (pseudo-hyperboloid) subreflector is used, the source which must be placed at the focus situated between the main reflector and the subreflector provides a degree of masking for the wave emitted or received by the antenna.
An object of the invention is therefore to solve these various problems.